Switch-mode power supplies produce a heavily pulsed load on the electrical power supply system, which leads to harmonic currents in electrical power supply systems. This load occurs in the region of the voltage maxima of the sinusoidal mains voltage, at which voltage maxima an energy-storage capacitor for the switch-mode power supply is recharged. Appliances having a relatively high power consumption, such as televisions with relatively large picture tubes, now therefore have to comply with specific regulations relating to harmonic currents. The harmonic load on the electrical power supply system caused by an appliance can in this case be stated as a so-called power factor.
Widely differing circuit concepts are known for improving the power factor, for example from DE-A-196 10 762, EP-A-0 700 145 and U.S. Pat. No. 5,986,898. These contain a second current path with a coil between the mains rectifier and the primary winding of the transformer, with the inductance of this coil acting like a current pump, which is controlled by a switching transistor in the switch-mode power supply and in consequence broadens the pulsed current flow of the switching-mode power supply.
A further possible way to improve the power factor of a switch-mode power supply is to use a coil in the input area of the switch-mode power supply. This coil is also referred to as a 50 Hz coil, a mains frequency coil or a power factor coil. In order to avoid confusion with other coils, the description therefore always uses the expression power factor coil for this coil.
However, this power factor coil has the disadvantage that the current flow in the power factor coil is interrupted suddenly when the mains switch is operated in order to switch off the appliance. The energy which is stored in the coil must be dissipated, however. Since the open mains switch represents the highest impedance in the circuit, a very high voltage is therefore developed across the switching contacts of the mains switch, and causes an arc. This causes the mains switch to age more quickly, and the switch represents a safety risk since, in the worst case, it becomes a potential fire source.
Other mains switches, in which the switching contacts open at a slow rate, are likewise at risk. In this case, although the voltage across the contacts is not very high, an arc is nevertheless produced, however, and burns until the end of the corresponding power supply system half-cycle. In this case as well, a large amount of energy is lost in the switch, which leads to rapid aging.